to see which credential each workflow is using, go to Settings → Credentials as admin, you’ll see all Box credentials listed there. then open each Box node in your workflows and the credential name is shown directly in the node config.
also worth checking how many Box credentials exist total — if there’s only one, all workflows are already sharing it and your migration is simpler than you think.
just don’t disable the old app until you’ve done at least one scheduled test run on the new credential, not just manual. scheduled runs sometimes use cached tokens and fail silently.